This invention relates generally to a corrosion resistant coating for an under platform region of a gas turbine engine turbine blade, methods for applying corrosion resistant coatings, methods for repairing gas turbine engine blades, and corrosion resistant articles.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the hot-section components of the engine. These components include the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases directly impinge. These components are subject to damage by oxidation and corrosive agents.
Many approaches have been used to increase the operating temperature limits and service lives of the turbine blades and vanes to their current levels. The composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example by providing the component with internal cooling passages through which cooling air is flowed. Another approach used to protect the hot-section components is to coat a portion of the surfaces with a protective coating such as an aluminum-containing coating. The protective coating oxidizes to produce an aluminum oxide protective layer that protects the underlying substrate.
Leakage and bleed air carry corrosive materials to the non-flowpath sides of turbine blades. Metal salts such as alkaline sulfate, sulfites, chlorides, carbonates, oxides, and other salt deposits resulting from ingested dirt, fly ash, volcanic ash, concrete dust, sand sea salt, etc., are a major source of the corrosion. Other elements in the bleed gas environment can also accelerate the corrosion. Alkaline sulfate corrosion in the temperature range and atmospheric region of interest results in pitting of the turbine blade substrate at temperatures typically starting around 1200° F. (649° C.).
Corrosion pitting has been identified as a cause of fatigue cracking initiation in certain gas engine turbine blades in the under platform region. In the art, the under platform region has been coated with a simple aluminide coating or a platinum aluminide (PtAl) coating. PtAl is the most common coating for the under platform region. Platinum plating control in the complex geometry of the under platform region of the dovetail is very difficult. Platinum aluminide coating is also expensive. Parts with complex coating requirements require difficult masking and in-process strip cycles in order to obtain the proper coating in certain areas and avoidance in other areas. Further, in severe operating conditions, it has been found that PtAl coatings are not sufficient to prevent the corrosion pitting and subsequent fatigue cracking from occurring in the under platform section. The oxidation and corrosion damage can lead to failure or premature removal and replacement of the turbine blades unless the damage is reduced or repaired.
Thus, it would be desirable to provide a coating system and method for protecting certain portions of the turbine blade from corrosion and oxidation.
So-called “silicon-modified” aluminides have been proposed as aluminiding compositions to provide an aluminum-rich region for superalloy substrates in, for example, U.S. Pat. No. 4,310,574 and U.S. Pat. No. 6,126,758. The silicon-modified aluminides may be formed from slurry coatings that can be sprayed or otherwise coated onto the substrate. The volatile components are then evaporated, and the aluminum-containing component can be heated in a manner that causes the aluminum and silicon to diffuse into the substrate surface.
There are advantages to using slurries for aluminiding the substrates. For example, slurries can be easily and economically prepared, and their aluminum content can be readily adjusted to meet the requirements for a particular substrate. Moreover, the slurries can be applied to the substrate by a number of different techniques, and their wetting ability helps to ensure relatively uniform aluminization.
Some aluminum-containing slurry compositions include chromate ions that are known to improve corrosion resistance. While these slurry compositions may be useful for some applications, the chromate ions are considered toxic. In particular, hexavalent chromate (Cr+6) is also considered to be a carcinogen. Thus, use of these types of coating compositions results in special handling procedures in order to satisfy health and safety regulations that can increase cost and decrease productivity.
U.S. Pat. No. 7,270,852 provides slurry-type aluminizing compositions for enriching the surface region of a metal-based substrate with aluminum. An exemplary composition includes colloidal silica and particles of an aluminum-based powder and is substantially free of hexavalent chromium.
It would be desirable to provide a suitable coatings and coating methods for the under platform regions of the turbine blades that avoid the difficulties encountered in the art. In particular it would be desirable to provide a coating that provides superior performance over known PtAl coatings, that is free of hexavalent chromium, that may be easily and uniformly applied, and which does not negatively impact the thermal mechanical properties of the underlying substrate.